iMac halts on boot
A few days ago, I think I restarted my iMac running macOS Sierra before it had finished closing down. At least, after that is when it started.
Now, when I boot the computer, the Apple logo comes up and then the white line goes about 1/3 of the way across before stopping. It just stays like that until I unplug. So far, when I replug, it has booted the entire way.
What can I do to fix the problem?
iMac 21.5", macOS 10.12